There are certain advantages (e.g., weight, transport, etc.) to forming relatively long, large-diameter articles from composite materials. Examples of such articles include a smoke stack, a tunnel liner, or an OTEC (“ocean thermal energy conversion”) cold-water pipe. Composite materials, which include a matrix constituent, typically a resin, and a reinforcement constituent, typically a fiber, are formed via a molding operation. But it is particularly difficult to fabricate long, large-diameter composite articles via molding processes. In particular, these articles are too long to be formed in a single “shot” (a single molding operation).
One approach to fabricating long, large-diameter articles is to mold a plurality of discrete relatively shorter-length sections and connect them via mechanical joints or adhesive bonding. Another approach is disclosed in U.S. patent application Ser. No. 12/484,779. Regardless of the approach taken, to achieve the shortest fabrication time and lowest cost, it is desirable to limit the number of molding shots required to form the article.
There are a number of manufacturing processes that are suitable for fabricating articles from composite materials. But few if any of those processes are well suited for the fabrication of extremely long, large diameter composite articles. It is notable that when molding planar parts (or articles comprised of a collection of planar parts), a horizontally oriented molding apparatus can and is used. But when molding non-planar parts, such as the cylindrical sections that would be required to form a smoke stack, etc., a vertically oriented molding apparatus must be used.
Perhaps the best option for fabricating long, large-diameter articles is a liquid-resin molding process. One liquid resin molding process is resin transfer molding (“RTM”). RTM uses a closed-cavity mold which is solid on all sides. In the simplest version of RTM, air is left in the fabric before resin injection. Some, but not all of this air is driven out through vents as the fabric fills with resin. In order to obtain an acceptable void content in the presence of this residual air, a very high pressure (about 275 psig) is sometimes applied while the resin is curing. The intent of the applied pressure is to shrink the size of any remaining air voids to acceptable levels. This large internal pressure generates substantial forces that tend to push opposing mold surfaces apart. For small molds, this problem is addressed using relatively inexpensive presses. But this approach becomes impractically expensive when dealing with large molds.
Another liquid resin process is vacuum-assisted resin transfer molding (“VARTM”). VARTM uses one-sided tooling. The term “tooling” or “tool” refers to a solid entity/surface against which the composite material is molded; it forms the shape of the molded article (“workpiece”) as the liquid resin transforms into a solid. FIG. 1A depicts a cross section of portion of a vertically oriented VARTM apparatus. In this example, the mold is forming a cylindrical workpiece and, as such, the molding region would actually be ring-shaped or annular, such that a top view of the apparatus would show a ring-like molding region.
With reference to FIG. 1A, the two constituents of the nascent composite—fabric 116 and resin 118—are shown within molding region 102. The liquid resin, which is introduced through inlet line 114, is retained on one side by hard tool 104 (i.e., the “one-sided” tooling) and on the other by soft tool 108 (e.g., a resilient layer, a vacuum bag, etc.). A gas, typically air at atmospheric pressure, is introduced through gas inlet 112 behind soft tool 108 into region 110 formed between wall 106 and the soft tool. With atmospheric pressure behind soft tool 108 and zero pressure in front of it (at least near the top of the resin), compaction pressure CNET is applied to the solidifying composite.
All of the resin being processed in the molding region is in liquid form. If the mold is vertically oriented, as in FIG. 1A, there would be a substantial gradient of hydrostatic pressure in resin 118 for tall workpieces. If the internal absolute pressure in front of soft tool 108 at the top of resin 118 is zero (it is under full vacuum), then the internal pressure near the bottom will be some pressure greater than zero (as a function of the resin's specific gravity and the height of the “column” of resin). If the product of the height of the resin column and the density of the resin are such that the hydrostatic pressure at the bottom of the resin column is greater than one atmosphere, the soft tool would billow outward (since the pressure behind the soft tool is only one atmosphere). If this were to happen, the soft tool would be not be applying any compaction pressure against the nascent composite.
Since the density of the liquid resin is similar to water, the aforementioned effect places an absolute limit of about 33 feet on the height of a workpiece that can be made using VARTM. For a column of resin 33 feet high and under full vacuum, there would be zero absolute pressure at top of resin and 1 atmosphere absolute pressure at the bottom. Referring to FIG. 1B, for a column of resin 33 feet high, there would be one atmosphere net compaction pressure CNET(top) at the top of the resin (i.e., atmospheric pressure minus zero hydrostatic pressure) and zero compaction pressure CNET(bot) at the bottom (i.e., atmospheric pressure minus 1 atm hydrostatic pressure). This would produce very non-uniform composite properties, such as, for example, fiber volume fraction. This second effect (non-uniform properties) limits the practical working heights for a single “shot” of VARTM to about 10-15 feet, wherein an acceptable compaction pressure can be maintained across the whole workpiece, as depicted in FIG. 2. But such a height-limited apparatus will still experience non-uniform compaction pressure, which will result in non-uniform properties of the resulting composite. Furthermore, depending upon the ultimate length of the article being fabricated, this approach might be impractical since the height limitation might require too many molding runs (i.e., shots).
Available molding processes are therefore poorly suited for the production of very long and very wide composite articles that need to be produced via a vertically oriented molding.